In an effort to control unwanted e-mail, some are turning to Challenge/Response, an email filtering system that checks incoming mail against a white list of approved addresses (such as your personal address book.) If the sender is not on the list, he is sent a challenge email with instructions on how to proceed. If the sender responds correctly, it is assumed he is neither a virus nor computer, and the message is passed to the intended recipient. For more information, see Mailblocks and “EarthLink to Offer Anti-Spam E-Mail System“.
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Feldman, Barbara. "Challenge/Response." Surfnetkids. Feldman Publishing. 15 Sep. 2003. Web. 23 Apr. 2014. <http://www.surfnetkids.com/tech/175/challengeresponse/ >.